Project Summary A central question in developmental biology is how biochemical signals and mechanical forces affect individual and collective cell behaviors to control tissue morphogenesis. With advances in imaging technologies and image analysis tools it is now possible to relate the dynamics of biochemical signals and force generating proteins with the deformation of cells and tissue domains and measure the forces that power cellular and tissue remodeling. Using live imaging and quantitative image analysis we discovered that contractile forces generated by contractile actomyosin networks affect cell shape changes and tissue remodeling of the apical epithelium of the fly retina. Unexpectedly, we found that the WAVE regulatory complex (WRC), the Arp2/3 complex and protrusive branched F-actin networks generate protrusive forces along apical junctions that expand cell-cell contacts and resist the impact of contractile forces during tissue remodeling. The mechanisms promoting protrusive dynamics at apical junctions during epithelial morphogenesis have been not characterized. Therefore, the goal of the proposal is to bridge this gap in knowledge. In Specific Aim 1 we propose to examine the role of the adhesion protein Sidekick in epithelial remodeling and the extent Sidekick physically targets the WRC to apical junctions. In Specific Aim 2 we propose to examine the role phosophoinositide PI(3,4,5)3 and Pten in epithelial tissue remodeling and the extent these components activate the WRC and coordinate protrusive with contractile dynamics at apical junctions. Previous models suggested that maximizing adhesion between eye cell types controls eye epithelial morphogenesis. Our data suggest that tensile forces with polarized distribution at apical junctions also play a role. In Specific Aim 3 we propose to measure the tension that cell-cell contact hold using laser ablation and correlate these measurements with the abundance of contractile MyoII and protrusive F-actin and with estimates of tension inferred using an inverse Cellular Vertex Model utilizing segmented images. Additionally, we will use the same approach to determine the relative contribution of the WRC and WRC regulators to the force balance. The completion of the Aims will elucidate mechanisms by which protrusive WRC-Arp2/3-based F-actin networks operate at apical junctions and coordinate with contractile actomyosin networks to control apical junctions remodeling, cellular morphogenesis and tissue remodeling.